(RISC-V fan here) This is a real-world use case. GMP is a library for handling huge integers, and adding two huge integers is one of the operations it performs

(RISC-V fan here) This is a real-world use case. GMP is a library for handling huge integers, and adding two huge integers is one of the operations it performs, and the way to do that is to add-with-carry one long sequence of word-sized integers into another. It's not synthetic; it's extremely specialized, but real.